Interconnect structures comprising metallization lines connect various components of semiconductor integrated circuits (ICs) both within each layer and in different layers. The metallization lines within each layer are formed in an interlayer dielectric (ILD) material. The ILD material electrically isolates metallization lines from one another within each level and in adjacent levels of interconnect structures. Damascene processes including single damascene process and dual-damascene process are routinely used for fabricating multi-level interconnect structures. In a damascene process, trenches and via holes are made inside and through an ILD layer, and filled with a conductive material, such as copper (Cu) or a Cu-based alloy, to create metallization lines and vertical conductive paths (vias) between adjacent layers.
Copper or copper-based alloy is used in metallization lines of semiconductor interconnect structures because of its higher electrical conductivity and higher resistance to electromigration compared to aluminum. However, copper may diffuse through an ILD layer and a semiconductor substrate to cause device reliability issues or poison transistors in a semiconductor IC device. As ICs and semiconductor devices get smaller, the size of interconnect structures also decreases. To prevent copper diffusion becomes increasingly important to improve reliability of semiconductor devices.